1. Field of the Invention
The present invention relates to an image processing apparatus suitable for use with a digital copying machine.
2. Description of the Prior Art
In a digital color copying machine, a latent image of one color is formed on a photosensitive member in one original document exposure cycle. In this case, light is irradiated uniformly on a high density area and a low density area of the original document. As shown in FIG. 1, if a signal from a photosensor such as a CCD is reproduced before a .gamma.-conversion (logarithmic conversion), the output signal from the CCD is non-linear relative to the original document density, that is, it lacks linearity. Accordingly, a reproduced image appears unnatural. In FIG. 1, the abscissa represents the original document density, and an ordinate represents the A/D converted value of the CCD output produced by irradiating the original document at an illumination of 20 luxes. P.sub.i represents an original document density versus A/D converted output curve before the .gamma.-conversion, and q.sub.i represents that after the .gamma.-conversion. In general, a dark area of the original document, that is, a shadow area has a gentle pre-conversion gradient, and a highlight area has a sharp pre-conversion gradient.
As a result, when the shadow area is .gamma.-converted, a change in the A/D converted output before the .gamma.-conversion for a given original document density change is one unit while a change after the .gamma.-conversion is five units. Accordingly, fine tonality cannot be reproduced.